A typical thermoforming machine includes a series of stages that transform thermoplastic material (e.g., polypropylene, polyvinyl chloride, polystyrene and others) into finished containers by means of heat and pressure. The finished containers are typically used to hold food, and may be formed into a variety of sizes and shapes depending on the thermoforming process.
Thermoplastic materials may be fed into a thermoforming machine in the form of a continuous sheet or as individually cut blanks or billets. When the input materials are individual thermoplastic billets, the process is called scrapless thermoforming because the finished containers need not be cut from the sheet after forming, reducing the amount of scrap material. In scrapless thermoforming, a billet feeding unit is typically used to load individual billets onto a conveying device in the machine.
Prior to being formed into containers, the billets must be heated to the desired temperature. The desired temperature depends on the structure of the machine being used as well as the desired properties of the end product. For example, containers may be formed while the thermoplastic material is below the crystalline melt point of the material, this being known as solid-phase pressure forming. Other methods involve heating the material to its melting point prior to forming, this being known as melt-phase thermoforming.
A conventional scrapless thermoforming machine has several stages used to create formed containers. First, the billets are loaded into the machine. Second, an oven is used to heat the billets to the desired temperature. Third, a form station or form press utilizes air pressure and vacuum to form the individual containers. After exiting the form press, the formed containers are removed from the machine at an unloading station. Other stations may be added to the thermoforming machine as desired, such as a pre-heating oven prior to entry into the main oven. A conveyor is typically used to transport the billets through the various stations.
Whether the containers are formed utilizing solid-phase forming or melt-phase forming, a problem that must be addressed involves the deformation of the thermoplastic material as it is heated. Because the billets are typically supported in a horizontal fashion with only peripheral supports beneath each billet due to the configuration of the form press, the billets tend to sag or droop between the supports when heated. In some cases, the billets may entirely fall out of their supports when heated due to the material deformation.
The thermoforming art has tried many methods of preventing material deformation from adversely impacting the thermoforming process. One method involves minimizing the size of the billets. This method prevents the formation of larger containers. Another method is to utilize round billets such that the resultant uniform deformation allows the support to retain the billet during heating. This method precludes the forming of containers having a non-uniform cross-section, such as rectangular or oval containers. Yet another method of addressing the deformation issue is to utilize a ring placed on top of the billet to strengthen the frictional grip on the billet during heating. This method presents additional manufacturing challenges with respect to how to efficiently place individual rings on top of each billet during the loading process.
An example of a prior art frame for holding a billet is U.S. Pat. No. 6,896,506 to Jordan, which describes a thermoforming machine that includes a loading station configured to load a thermoplastic billet onto a frame. The frame has an outer ring and a billet support blade defining a channel therebetween. The billet support blade engages the billet to maintain the billet on the frame during heating of the billet. A heating apparatus is configured to heat the billet and a forming station is configured to form the billet into a container.